The present invention relates generally to center electrode assemblies for spark plugs and igniters and, more particularly, to multi-piece center electrode assemblies that utilize an iridium-based firing tip.
Over the years, a variety of erosion resistant metals and alloys have been employed in center electrode assemblies to increase the life of igniters and spark plugs used in turbine and piston engines. Metals such as platinum and iridium have proven to exhibit improved consumption resistance and ignitability. Because of their expense, these noble metals are not typically used for the entire center electrode, but rather are used only to form a firing tip at the working end of the center electrode where the spark is produced. In this way, erosion at the firing surfaces can be reduced without the expense of forming the entire center electrode from the more expensive metal. Apart from their cost, most of these metals have other inherent drawbacks that complicate their use as a firing tip. For instance, platinum has a relatively low melting point (1769xc2x0 C.) and while iridium has a much higher melting temperature (2454xc2x0 C.), it has poor weld characteristics and is a brittle metal that exhibits axial cracking when subjected to deformation or significant pressure.
An example of the use of iridium in conjunction with a spark plug center electrode can be seen in U.S. Pat. No. 5,557,158 issued to Kanao et al. The Kanao patent discloses a spark plug center electrode having a cylindrical or ringlike iridium insert mechanically attached to the end of the electrode. The iridium insert is attached to the electrode by welding or by otherwise securing a stopper component to the very tip of the center electrode, thereby securing the iridium insert between the two components and preventing it from being dislodged.
The poor weld characteristics of iridium make it difficult to metallurgically attach iridium firing tips to other electrode components. Similarly, iridium has not be considered a good candidate for swaging or otherwise mechanically attaching it to the center wire because of the potential for cracking. However, where more traditional (non-iridium) metals have been used, various approaches have been proposed for mechanically connecting center electrode components together. For example, U.S. Pat. No. 1,344,954 to Meyer discloses a two-piece spark plug center electrode assembly wherein a lower electrode member having an expanded head section is received by inwardly biased retaining jaws of an upper electrode member. The axial location at which the retaining jaws contact the expanded head section is variable depending on the axial position of the upper member, which may be adjusted via threads located at its upper end. Thus, the two electrode members are mechanically secured to each other through the radially-inward pressure imparted by the retaining jaws against the expanded head section.
Similarly, U.S. Pat. No. 1,633,435 to De Alcocer et al. discloses a two-piece spark plug center electrode assembly in which the stem of a lower electrode member is inserted into a receiving element of an upper electrode member. The upper member is threaded at one axial end and terminates into a hollow conical head section for receiving the stem of the lower member at the other axial end. Downward axial movement of the upper member against the lower member, by the tightening of a nut over the threaded portion of the upper member, causes the conical head section to contract over the lower member stem, thereby securing the two members together.
U.S. Pat. No. 6,069,434 to Clifford discloses a three-piece spark plug center electrode assembly wherein input and emitter electrodes are coupled together via an interconnector. The interconnector is a hollow cylindrical body having a series of indentations located on its surface for gripping the input and emitter electrodes. In assembly, the input and emitter electrodes are inserted into opposite ends of the interconnector, and the interconnector is crimped around the electrodes such that the indentations grasp and retain the input and emitter in place. Because the input and emitter members do not physically contact each other, the interconnector serves as both a mechanical and electrical connector.
None of these latter patents are directed to the problems associated with use of a multi-piece center electrode that uses an iridium electrode member at the firing end of the center electrode assembly. There is thus a need for a center electrode assembly that provides a connection between the iridium and other electrode component that is both secure from separation and that does not damage the structural integrity of the iridium during the attachment process.
The above-noted shortcomings of prior art center electrode assemblies are overcome by the present invention which provides a center electrode assembly for use in an ignition device such as a spark plug or igniter. The center electrode assembly includes elongated first and second electrode members. The first electrode member has first and second axial ends with a hole extending into the second axial end. The second electrode member comprises iridium and has third and fourth axial ends and a head section located at the third axial end. The head section is positioned within the hole of the first electrode member such that the second axial end of the first electrode member is swaged over the head section, thereby mechanically securing the first electrode member to the second electrode member. Preferably, the head section comprises a radially expanded head that is not only connected to the first electrode member by swaging of the second axial end over the head section, but also by brazing of the head section to the first electrode member within the hole. The center electrode assembly constructed in this manner can be incorporated into an ignition device that also includes a shell having an axial bore, a ground electrode connected at a lower axial end of the shell, and an insulator secured within the shell""s axial bore. The center electrode assembly is located within a central bore of the insulator at its lower axial end.
In accordance with another aspect of the invention, there is provided a method for producing a multi-piece center electrode assembly for use in an ignition device. The method comprises the steps of:
(a) providing an elongated first electrode member having a hole located at an axial end,
(b) providing an elongated second electrode member having a radially expanded head section located at an axial end,
(c) introducing brazing compound into the blind hole,
(d) inserting the head section into the blind hole,
(e) swaging the axial end of the first electrode member around the head section, and
(f) heating the brazing compound to a temperature sufficient to braze the first and second electrode members together.
Preferably, the second electrode member comprises iridium so that, by swaging the non-iridium electrode member over the iridium electrode member and then heating the assembled components to brazing temperatures, the electrodes are both mechanically and metallurgically connected together in a manner that helps maintain the integrity of the braze joint over time while not causing any significant axial cracking of the iridium that could otherwise occur if it were to be deformed.
Objects, features, and advantages of the invention include providing an iridium tipped multi-piece electrode assembly for an ignition device that allows for the two electrode members to be securely fastened together without compromising the structural and electrical integrity of the lower iridium member. Additionally, the invention enables the center electrode assembly to be designed in a manner that is economical to manufacture and assemble and that can provide a relatively long and useful life in service.